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…sk, or count expr2tags_Unop_DC now takes the operation. Unary operations whose result is a boolean, or a mask or count computed from the operand's bits (CmpNEZ*, CmpwNEZ*, 32to1, 64to1, Clz*, Ctz*, PopCount*, GetMSBs*), give the result tag 0, as binary comparisons do. All other unary operations return the tag of the operand, as before. The operand's tag is passed to MC_(helperc_RETURN_0) rather than discarded. Returning a constant tag would let VEX remove the operand's tag expression as dead code, along with any merge it performed. For example, in 'char lt = (a < b)' the merge of a and b would be lost.
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…perands Comparisons merge their operands' tags with a dirty call, so VEX cannot remove the merge when a later operation discards the comparison's tag. Unary operations that give tag 0 anchor their operand's tag with a dirty MC_(helperc_TAG_NOP) call (except in the dataflow modes) and return constant 0, replacing MC_(helperc_RETURN_0). Iop_32to1 and Iop_64to1 return the operand's tag again, because they also extract data bits. Iop_Cnt* and Iop_Cls* give tag 0. Add a regression test, dyncomp-unop-tag0-test.sh.
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Review comments at @valgrind/fjalar/tests/dyncomp-unop-tag0.goal:
- Around line 1-12: Update the test case for ctz_of_sum so one operand has a
nonzero tag established independently of i before the call. Avoid deriving both
operands from i or relying on the literal 5 or initially constant r to provide
that tag.
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Binary comparisons merge their operands' tags and give the result tag 0, because the result is a boolean. With this pull request, the unary operations
CmpNEZ*andCmpwNEZ*(booleans or lane masks) andClz*,Cls*,Cnt*,Ctz*,PopCount*, andGetMSBs*(counts or masks of the operand's bits) do the same, instead of passing the operand's tag through. To support this,expr2tags_Unop_DCnow takes the operation and switches on it. Every other unary operation reaches thedefaultcase, which returns the operand's tag as before. That includes32to1and64to1, which can extract a data bit rather than convert a boolean.These unary operations discard the operand's tag, which would let VEX remove the operand's tag computation as dead code, together with any clean-call merge in that computation (such as the merge of
aandbinctz(a + b)). To prevent that, they anchor the operand's tag with a dirty call to the existingMC_(helperc_TAG_NOP), then return constant 0. The anchor is omitted for constant operands and in the dataflow modes, where no binary operation merges with a clean call.For the same reason, a binary comparison now merges its operands' tags with a dirty call to
MC_(helperc_MERGE_TAGS_RETURN_0), ashandleCCall_DCalready did, rather than with a clean call. Otherwise, VEX would remove the merge whenever a later operation discards the comparison's tag, such as the shift amount inx << (a < b).New test
valgrind/fjalar/tests/dyncomp-unop-tag0-test.sh, run bymake fjalar-test, checks the resulting comparability.This is one of three pull requests split from #83. It is independent of #85. #87 builds on both.